FIG. 1 shows an example of conventional strain sensing apparatus. This apparatus is shown in Japanese Published Patent Application No. SHO 59-188968. Referring to FIG. 1, magnetic layers 2a and 2b are fixed by, for example, bonding to a shaft 1 on which external force is exerted. It is assumed that the magnetic layer 2a has a magnetic anisotropy lying along +45.degree., while the magnetic layer 2b has a magnetic anisotropy lying along -45.degree.. Cylindrical sensing coils 3a and 3b are disposed to face and surround the magnetic layers 2a and 2b, respectively, with a spacing provided therebetween. An exciting coil 4 common to the sensing coils 3a and 3b is disposed to surround the sensing coils 3a and 3b. The exciting coil 4 is driven with current supplied from a driving power supply 5. Output voltages of the sensing coils 3a and 3b are applied to smoothing circuits 6a and 6b, respectively. Output voltages of the smoothing circuits 6a and 6b are applied to a differential amplifier circuit 7 for differential amplification. An output voltage from the differential amplifier 7 is derived from an output terminal 8. This output voltage at the output terminal 8 is a strain representative signal which represents a magnitude of strain of the shaft 1.
The exciting coil 4 driven with current from the driving power supply 5 generates magnetic flux which passes through the magnetic layers 2a and 2b. A strain is generated in the shaft 1 in accordance with an external force applied to the shaft 1, and the permeabilities of the magnetic layers 2a and 2b change in opposite directions. Changes in the permeability of the magnetic layers 2a and 2b cause changes in the mutal inductance between the exciting coil 4 and the sensing coils 3a and 3b. This causes changes of the voltages induced in the respective sensing coils 3a and 3b. The changes of the induced voltages are smoothed in the respective smoothing circuits 6a and 6b and, then, differentially amplified in the differential amplifier circuit 7. The output voltage from the differential amplifier circuit 7 is developed at the output terminal 8 as a signal representative of a magnitude of strain of the shaft 1. From this output voltage, the amount of strain of the shaft 1 can be determined.
FIG. 2 shows another example of conventional strain sensing apparatus. The apparatus is shown in Japanese Published Patent Application No. SHO 59-166828. In FIG. 2, magnetic layers 12a and 12b are secured, by means of, for example, annular plated films 100, to a shaft 11 to which external force is exerted. Alternatively, the magnetic layers 12a and 12b may be bonded by an adhesive to the shaft 11. As in the case of the layers 2a and 2b of FIG. 1, the magnetic layers 12a and 12b have magnetic anisotropy lying along +45.degree. and -45.degree., respectively. Cylindrical sensing coils 13a and 13b are disposed to face and surround the magnetic layers 12a and 12b. One terminal of each of the sensing coils 13a and 13b is connected to a voltage source V.sub.cc. The other terminals of the coils 13a and 13b are connected to the collectors of transistors 25 and 26, respectively. The emitters of the transistors 25 and 26 are connected to output terminals 27a and 27b, respectively. The sensing coils 13a and 13b and the transistors 25 and 26, together with resistors 20-23, a variable resistor 24 and capacitors 18 and 19 form a multivibrator circuit. A capacitor 17 forms a smoothing circuit. The multivibrator circuit and the smoothing circuit form a magnetostriction sensing circuit 16.
The sensing coils 13a and 13b of the strain sensing apparatus of FIG. 2 are driven with current supplied from the driving voltage source V.sub.cc and, therefore, act also as exciting coils which generate magnetic fields. The variable resistor 24 is adjusted to provide such a value that when the shaft 11 has no strain, equal maximum collector currents (or emitter currents) I.sub.C1 and I.sub.C2 flow in the transistors 25 and 26 which are alternately turned on and off by the multivibrator action. Then, a voltage developed between the output terminals 27a and 27b is OV. When a strain is generated in the shaft 11, the permeabilities of the magnetic layers 12a and 12b change. The directions of the permeability changes, however, are opposite to each other. The changes in the permeability of the magnetic layers 12a and 12b cause changes of the self-inductances of the sensing coils 13a and 13b, respectively. This breaks the equilibrium condition of the circuit, and the respective collector (or emitter) currents I.sub.C1 and I.sub.C2 and the duty cycle or oscillation frequency of the multivibrator circuit change. This, in turn, causes a DC voltage proportional to the magnitude of the strain of the shaft 11 to be developed across the smoothing capacitor 17 and, hence, between the output terminals 27a and 27b. From this DC voltage, the magnitude of the strain can be determined.
The conventional strain sensing apparatus shown in FIG. 1 has a disadvantage that since it uses two coil arrangements, namely, the sensing coils 3a, 3b and the exciting coil 4, fabrication and adjustment of the coils requires much time and, furthermore, wiring is also time-consuming.
The disadvantage of the apparatus of FIG. 1 can be eliminated in the conventional apparatus of FIG. 2. However, since the same circuit is used for both field generation and permeability sensing, it is difficult to adjust circuit parameters, such as an oscillation frequency and current values, to appropriate values. Furthermore, since the strain sensing apparatus of FIG. 2 can be effective only in the first quadrant of the B-H curve of the magnetic layers 12a and 12b, it can operate only on a portion of the B-H curve loop. Accordingly, the performance of the apparatus is much affected by a shift of the operating point which may be caused by disturbance fields. In other words, this apparatus is less resistant to external disturbance fields. Another disadvantage of this apparatus is that its sensitivity is low. These facts will be described in detail later with reference to FIGS. 3-7.
FIG. 3 shows a major portion of the multivibrator circuit of the strain sensing apparatus of FIG. 2. The same reference numerals as used in FIG. 2 designate the same items. The transistors 25 and 26 act as switching elements. The collector currents I.sub.C1 and I.sub.C2 of the transistors 25 and 26 flow only in one direction, varying as shown in FIG. 4. Since these currents I.sub.C1 and I.sub.C2 flow also through the sensing coils 13a and 13b, the operating range of the magnetic layers 12a and 12b is as indicated by a thick line P.sub.1 in an operating field region H.sub.a of the B-H curve as shown in FIG. 5. The portion P.sub.1 is along a loop lying somewhat inward of the major loop of the B-H curve and can be considered to be a minor loop of the B-H curve.
When a disturbance magnetic field H.sub.d &gt;0 acts on the magnetic layers 12a and 12b, the operating magnetic field region H.sub.a shifts in the positive direction from the origin by H.sub.d, as shown in FIG. 6, and, then, the operating range of the magnetic layers 12a and 12b becomes one indicated by thick lines P.sub.2. If disturbance field H.sub.d acting on the layers 12a and 12b is less than O (H.sub.d &lt;0), the f field acting on the layers 12a and 12b is -H.sub.d .about.(H.sub.a -H.sub.d) and the field operating region H.sub.a shifts toward the minus (-) region as shown in FIG. 7, so that the operating range becomes a minor loop represented by a thick line portion P.sub.1, which is in the range of H.sub.a.
As described above, although the apparatus of FIG. 2 is free of disadvantages of the apparatus of FIG. 1, it has a disadvantage that due to disturbance magnetic fields, its operating point changes greatly, which causes sensitivity reduction and offset variations.
Offset variation is a ratio of the magnitude of shift of the detection zero point of the sensor due to the presence of disturbance field, to the strain detecting sensitivity of the sensor in the absence of disturbance field, and can be defined by the following expression. EQU Offset Variation=.vertline.V-V.sub.Hd=0 .vertline./Gain.sub.Hd=0 .times.100(%)
where V is the sensor output under a condition in which a disturbance magnetic field is present and no torque is applied to the shaft, V.sub.Hd=0 is the sensor output under a condition in which no disturbance field is present and no torque is applied to the shaft, and Gain.sub.Hd=0 is the distortion detecting sensitivity under a normal operating condition in which a magnetic field is not applied and torque is applied to the shaft.
The present invention can eliminate drawbacks present in conventional strain sensing apparatus, such as the ones described above. According to the present invention, a strain sensing apparatus is provided, in which driving parameters for sensing coils can be relatively freely set and which has a sensing characteristic that is highly stable against external disturbance magnetic fields.